This invention relates to a recording element having a spin-valve structure and to a storage device, and more specifically to a spin-valve recording cell structure capable of information writing and reading by controlling the direction of the magnetization vector of a magnetized ferromagnetic layer, and to a storage device utilizing such a recording cell structure.
In recent years, advances in nanoelectronics have been accompanied by product development in fields applying physical phenomena specific to minute-sized magnetic materials. Of these fields, there have been particularly rapid advances in the field which utilizes the spin of free electrons in magnetic materials (hereafter called “spin electronics”).
In spin electronics, devices currently thought to have the highest possibility of practical application are spin-valve elements which apply the tunneling magnetoresistance (TMR) effect occurring in a layered structure of a ferromagnetic layer/insulating layer/ferromagnetic layer or the giant magnetoresistance (GMR) effect occurring in a layered structure of a ferromagnetic layer/nonmagnetic layer (conductive layer)/ferromagnetic layer. The slash (/) symbol appearing between layers indicates that the preceding and following layers are layered in the order indicated.
FIG. 1 and FIG. 2 show examples of the configurations of spin-valve elements of the prior art. Of these, FIG. 1 shows the basic configuration portion of a spin-valve element utilizing TMR. This element comprises, configured on a substrate 5, one insulating layer 24, a pair of ferromagnetic layers 23 (fixed layer) and 25 (free layer) sandwiching the insulating layer 24, and electrode layers 21 and 27, with an antiferromagnetic layer (pinning layer) 22, a capping layer 26, and similar structures added as necessary. The magnetization of the fixed layer 23 is fixed by magnetic coupling with the antiferromagnetic layer 22 and similar structures. The magnetization of the free layer 25 is controlled by spin injection by an external magnetic field or by a spin-polarized current. Reference number 30 designates an insulating element and reference number 31 designates wiring.
In control by spin injection, when electrons are passed to this element from the fixed layer 23, a torque acts so as to make the spin of the free layer 25 parallel to that of the fixed layer 23. Conversely, when electrons are passed from the free layer 25 toward the fixed layer 23, a torque acts so as to make the spin of the free layer 25 antiparallel to that of the fixed layer 23. Through this action, the magnetization direction of the free layer 25 can be controlled through the current direction (magnetization reversal by spin injection).
In this way, by causing the direction of the magnetization of the free layer and fixed layer to rotate or reverse according to spin injection by an external magnetic field or a spin-polarized current, the resistance of the element changes greatly according to the magnetization directions of both. That is, an element is configured such that when the magnetization vectors of both are antiparallel, the highest resistance value is obtained, and when the magnetization vectors of both are parallel, the lowest resistance value is obtained.
FIG. 2 shows the basic configuration portion of a spin-valve element utilizing GMR. A difference with the element utilizing TMR of FIG. 1 is that the insulating layer (conductive layer) 24 is replaced with a nonmagnetic layer 51; otherwise the functions are essentially the same.
Using these elements, solid-state magnetic memory devices which record one bit of information per element have been proposed, and multivalue recording techniques capable of recording two bits of information per element have also been proposed. Two states (two values) can be recorded for one bit, and four states (four values) can be recorded for two bits.
Patent Reference 1: Japanese Patent Application Laid-open No. H10-91925.
Patent Reference 2: Japanese Patent Application Laid-open No. 2003-31771.
Patent Reference 3: Japanese Patent Application Laid-open No. 2007-317895.
Patent Reference 4: Japanese Translation of PCT Application No. 2005-535111.
Non-patent Reference 1: B. A. Ivanov et al, “Excitation of Spin Dynamics by Spin-Polarized Current in Vortex State Magnetic Disks”, Phys. Rev. Lett. No. 99, 247208, The American Physical Society, Dec. 14, 2007.
Non-patent Reference 2: J. Shibata et al, “Current induced magnetic vortex motion by spin-transfer torque”, Phys. Rev. B., No. 73, 020403, The American Physical Society, Jan. 4, 2006.
However, there are various problems to be solved in technology for multivalue recording of the prior art, and commercialization has not yet been attained.
For example, Patent Reference 1 (Japanese Patent Application Laid-open No. H10-91925) proposes a solid-state magnetic memory device employing a multivalue recording technique capable of storing two bits of information per element, obtained by forming a dual-structure tunnel junction element having the layered structure of a ferromagnetic layer/first insulating layer (or first nonmagnetic layer)/ferromagnetic layer/second insulating layer (or second nonmagnetic layer)/ferromagnetic layer. In this solid-state magnetic memory device, a multiple-structure element structure is necessary, and between the layered configuration of a ferromagnetic layer/first nonmagnetic layer/ferromagnetic layer and the layered configuration of a ferromagnetic layer/second nonmagnetic layer/ferromagnetic layer, the level of the output voltage detects across the ferromagnetic layers of each is differentiated and detected. Hence there is the problem that in the solid-state magnetic memory device of Patent Reference 1, if the magnetoresistance exhibited by at least one among the two layered configurations contained in the solid-state magnetic memory device is not larger than the magnetoresistance exhibited by the ferromagnetic layer/nonmagnetic layer/ferromagnetic layer single-structure tunnel junction TMR element (the other TMR element), an adequate S/N ratio cannot be secured.
Further, Patent Reference 2 (Japanese Patent Application Laid-open No. 2003-31771) discloses a method in which two ferromagnetic layers are layered with a nonmagnetic layer intervening, formed such that the magnetization directions of the two ferromagnetic layers are mutually perpendicular, and by means of this combination one bit can be stored in each of the ferromagnetic layers, so that in both a total of two bits in four states can be stored independently. In this method, there are the problems that a multiple structure is necessary, and that moreover by generating magnetic fields in both forward and reverse directions for each of the magnetic fields, for a total of four directions, so that switching using an external magnetic field must be performed.
Further, Patent Reference 3 (Japanese Patent Application Laid-open No. 2007-317895) discloses a structure in which two standby portions are made adjacent to a free layer, and which has a notch which pins domain walls corresponding to each of the standby portions. In this method, there is the problem that because the lateral-direction area is large in the standby portions only, even if multivalue recording is realized, the recording density is not improved.
And, Patent Reference 4 (Japanese Translation of PCT Application No. 2005-535111) discloses a free layer which has a plurality of stable positions due to a shape anisotropy of the free layer; but there is the problem that in order to induce the shape anisotropy, the shape becomes irregular, and thus the reversal magnetic field is increased.